Isotope imaging devices such as the positron scintillation camera and the Mark II whole body scanner provide the means to compare new, easily imaged, gamma-emitting isotopes to the useful, reliable, but not readily imaged positron-emitting isotopes of Fe-52 (erythropoietic marrow) and F-18 (bone blood flow). Our findings that Indium-111 chloride may give false positive and/or false negative erythrpoietic marrow distribution patterns, demonstrated unequivocally in the same patient with Fe-52, warrant more intensive investigation of the nature of the discrepancy, and a description of the type of patient whose In- 111 bone marrow scan must be evaluated with caution. Tc-labeled phosphates, such as diphosphonate (EHDP) and polyphosphate, have become $ $stablished as superb substitutes for F-18 as bone scanning agents. We have pointed out that EHDP is also useful in assessing skeletal hyperperfusion in patients with myelofibrosis-osteosclerosis-myeloid metaplasia syndrome, and we plan to further document this observation. Direct intramedullary injection of a mixture of two istopes into the femur of anesthetized dogs has provided valuable information about the physiology and microcirculation of bone marrow. Further detailed analysis of data already collected will be made, with definitive animal experimentation as required.